The invention relates to a process for the thermal cracking of a charge incorporating at least one hydrocarbon containing at least one carbon atom, in a cracking or pyrolysis zone made from a particular alloy.
The term thermal cracking is understood to mean the processes of steam cracking a charge of at least one hydrocarbon having two carbon atoms in the presence of steam, thermal dehydrogenation processes such as methane pyrolysis, or catalytic dehydrogenation such as the dehydrogenation of ethyl benzene into styrene or propane into propylene. All these processes involve reactions with a high heat flow, followed by a rapid quenching of the pyrolysis effluents.
Throughout the remainder of the description, in a purely illustrative manner, the invention will be described as a process for the steam cracking of at least one hydrocarbon containing at least two carbon atoms and intended to produce light olefins.
Its principle is based on the instability at high temperatures of paraffins and naphthenes when compared with olefins and aromatics. The main reactions are the breaking of a C--C bond by a homolytic breaking mechanism in order to lead to an olefin and a paraffin, as well as dehydrogenation. These two reactions are endothermic and are therefore aided by a temperature rise. They also lead to an increase in the number of molecules, so that they are aided by low partial pressures of the hydrocarbons to be treated. This is why the said pressure is reduced to the maximum by adding steam to the reaction medium.
The problems linked with the use of steam cracking reveal several main sources of difficulties inherent in the process operating at high temperature, such as the oxidation of the constituent materials of the reactor, the cementation of these materials, the formation of coke on the walls in contact with the hydrocarbons and the creep behavior of the reactor.
One of the solutions proposed in WO 8700546 is the use of ceramic materials able to withstand very high temperatures (e.g., 1200.degree. C.), while obtaining a high heat flow. The latter factor can essentially be obtained by increasing the skin temperature of the tubular reactors and/or by decreasing the diameter of the tubes (which makes it possible to increase the s/v ratio, s being the exchange surface and v the reaction volume).
However, certain difficulties are encountered particularly with respect to sealing in contact with the ceramic forming the reactor and the metal supply and discharge tubes for the various fluids used in the process.
Advances made in metallurgy in connection with special alloys able to resist ever increasing temperatures (e.g. INCOLOY 800H, HK 40, HP40) have made it possible for steam cracking pyrolysis furnace designers to increase the operating temperatures of these tubular furnaces, the present limits being at approximately 1100.degree. C.
It is also known that in order to be able to obtain a better behavior under high temperature conditions of nickel-based metallic alloys, their nickel content should be greatly increased. Moreover, it is also known that nickel and iron catalyze the coke formation reaction on the reactor walls in contact with the charge.
In order to carry out a steam cracking reaction, account must be taken of the fact that the outer wall of the reaction zone in contact with the high temperature heating fluid is exposed to an oxidizing atmosphere, whereas its inner wall in contact with the charge is exposed to an overall reducing atmosphere. It is therefore vital that the material chosen has a good thermal behavior under these two extreme media.
U.S. Pat. No. 4,671,931 describes an alloy having a high nickel content and which resists high temperature oxidation. However, this patent makes no reference to the use of this alloy under severe conditions due to the simultaneous exposure to an oxidizing atmosphere and to a reducing atmosphere of hydrocarbons such as those encountered in thermal cracking processes. It makes no reference to the problem of using metallic alloys which are affected by cementation or to the problem linked with the coke formation speed.